Linear polyesters, such as poly(ethylene terephthalate) or PET, and poly(butylene terephthalate) or PBT, are widely used in the preparation of articles by forming methods such as injection molding. Many of their properties, including chemical stability, solvent resistance and low permeability to gases, make them attractive candidates for such forming operations as blow molding, profile extrusion and thermoforming. One problem in such operations is the relatively low melt viscosities of the polyesters, as a result of which the formed articles do not adequately retain their shape immediately after forming and before they have cooled.
In recent years, methods have been developed for increasing the melt viscosities and melt strengths of such polyesters. For example, Japanese Kokai 75/96648 and Japanese Kokai 81/116749 describe the reaction of various thermoplastic polyesters with triglycidyl isocyanurate (TGIC). In this reaction, a branched polyester is apparently formed by reaction of carboxylic acid end groups of the polyester with each of the epoxy groups. By reason of this branching, the polyester displays low viscosity at high shear rates, as during extrusion, and high viscosity and melt elasticity at low shear rates, as during hanging of a blow-molded parison, which ensures dimensional stability.
More recently, improved methods for conducting this reaction have been developed. U.S. patent application, Ser. No. 185,534, filed Apr. 21, 1988, describes the initial preparation of a reactive concentrate by the reaction of a relatively large amount of the polyepoxide with a linear polyester having a measurable proportion of free carboxylic acid end groups. The reactive concentrate is then melt blended with further linear polyester to form the desired branched polyester product. A somewhat different method, employing a concentrate in which the polymer is non-reactive with the polyepoxide, is the subject of U.S. patent application Ser. No. 125,859, filed Nov. 27, 1987. Both applications are copending and commonly owned herewith.
Kodama et al., U.S. Pat. No. 4,141,882 describes obtaining a polyester composition having high melt viscosity by blending a polyester comprising at least 80 percent by mole of ethylene terephthalate units with (1) an epoxy compound (A) and at least one organic compound (B) capable of reacting with the epoxy compound (A), or (2) a product of the melt reaction of epoxy compound (A) with the organic compound (B).
Blaschke et al, United Kingdom Patent No. 2,098,231 describe shaped bodies formed of polytetramethylene terephthalate stabilized with TGIC. The patentees also teach the addition of hindered phenolic stabilizers to the polyester compositions.
Commonly assigned, U.S. patent application Ser. No. 07/322,471, filed Mar. 3, 1989, now allowed discloses adding an effective amount of a catalyst compound to a polyester and polyepoxy compound composition to increase the melt viscosity of the polyester polyepoxy compound composition. However, no suggestion is made that still further melt viscosity enhancement can be obtained by adding an additional catalyst compound to a polyester composition comprising polyepoxy compounds and a hindered phenolic thermal stabilizer.
It has now unexpectedly been found that products with still better viscosity properties are obtained where at least one additional catalyst compound is added to a polyester composition comprising a polyester resin, a polyepoxy compound and a hindered phenolic thermal stabilizer. Surprisingly the addition of the additional catalyst compound significantly enhances the melt viscosity and melt strength of polyester compositions. This also enables the producer to obtain a product resin of equivalent melt viscosity while employing less polyepoxy compound.